Chemical analysis performed by the science instruments mounted on the rover's robotic arm measured a composition of about 90 percent pure silica — a material commonly found in quartz on Earth — for the bit of Martian dirt, said mission scientists, who first heard of the find during a teleconference.

"You could hear people gasp in astonishment," said Steve Squyres, principal investigator for NASA's twin Spirit and Opportunity rovers at Cornell University in Ithaca, N.Y. "This is a remarkable discovery."

Draggin' the line
This discovery came about unexpectedly as the result of a mechanical failure.

Both Spirit and its twin rover Opportunity completed their original three-month missions in April 2004, and are aging. One of Spirit's six wheels no longer rotates, gouging a deep impression as it drags through soil. That scraping has exposed several patches of bright soil, leading to some of Spirit's biggest discoveries at its Gusev Crater exploration site, including the most recent find.

Spirit had previously found other indicators of long-ago water at the site, such as patches of water-bearing, sulfur-rich soil, alteration of minerals, and evidence of explosive volcanism.

One possible origin for the silica may have been interaction of soil with acid vapors produced by volcanic activity in the presence of water, though the material could also have formed in water in a hot spring environment, NASA said in a statement. The latest discovery adds compelling new evidence for ancient conditions that might have been favorable for life, the space agency added.

Researchers informally dubbed the newly exposed patch of soil "Gertrude Weise," after a player in the All-American Girls Professional Baseball League.

"We've looked at dozens of disturbed soil targets in the rover tracks, and this is the first one that shows a high silica signature," said Steve Ruff of Arizona State University in Tempe, who first proposed using Spirit's thermal emission spectrometer to study the overturned soil's mineral composition last month.

Heart of glass
The silica readings in the overturned soil prompted mission managers to drive Spirit close enough to touch the soil with the rover's alpha particle X-ray spectrometer. Silica commonly occurs on Earth as the mineral quartz, and is the main component of window glass, NASA said. The Martian silica at the Gertrude Weise patch is non-crystalline, with no detectable quartz, the agency added.

Spirit worked within about 50 yards (meters) of the Gertrude Weise area for more than 18 months before the discovery was made.

"This is a target-rich environment, and it is a good thing we didn't go hurrying through it," said Squyres.